Supramolecular porphyrin as an improved photocatalyst for chloroform decomposition

In this work, the outlying decoration of the free-base meso-(4-tetra) pyridyl porphyrin (H2TPyP) with the RuCl(dppb)(5,5′-Me-bipy) ruthenium complex (here named Supra-H2TPyP) is observed as an improved molecular photocatalyst for dye-mediated chloroform (CHCl3) decomposition via one-photon absorption operating in the visible spectral range (532 nm and 645 nm). Supra-H2TPyP offers a better option for CHCl3 photodecomposition when compared to the same process mediated by pristine H2TPyP, which requires either excited-state- or UV absorption. The chloroform photodecomposition rates for Supra-H2TPyP as well as its excitation mechanisms are explored as a function of distinct laser irradiation conditions.


Samples and spectroscopic measurements
The synthesis of H 2 TPyP and its supramolecular H 2 TPyP [RuCl(dppb)(5,5 ′ -Me-bipy)] 4 structure (Supra-H 2 TPyP, see Fig. 1) follows the procedures described in the literature. 47,57 The spectroscopic measurements were conducted with samples dissolved in CHCl 3 stabilized with amylene (NEON Inc.) used as received. Solution concentrations were kept below 10.0 mM to prevent spontaneous aggregation and inner lter effects. Absorption spectra were acquired with a JASCO V-670 spectrophotometer whereas steady-state photoluminescence (PL) spectra were measured using a setup composed of a Xenon lamp (ACTON); a monochromator (ACTON, model 300i) and a portable spectrophotometer (Ocean Optics), with the PL signal being detected in a 90°geometry relative to the excitation beam direction. Time-resolved PL experiments were conducted using a Time-Correlated Single Photon Counting (TCSPC) system (Horiba, Delta-Flex model with 27 ps of temporal resolution), equipped with a pulsed excitation source (l exc = 352 nm with 8.0 MHZ of repetition rate). The PL decays were collected at the maximum of each spectrum. Emission quantum yields 58,59 were calculated adopting H 2 TPyP dissolved in CHCl 3 as standard 22 and applying eqn (1): where, F sa and F st stand for the quantum yields of the sample (sa) and standard (st) solution, respectively. The quantities Abs st (F st ) and Abs sa (F sa ) are the absorbances at 420 nm (the integrated PL spectrum for the corresponding excitation) from the sample and standard solutions.  58 From the Lambert-Beer law, 58 F T F I ¼ 10 ÀAðlÞ (with A(l) corresponding to absorbance at the incident wavelength, for the non-irradiated solution) resulting in eqn (2):

Photochemistry assays
During all the spectroscopic measurements and irradiation processes, the samples were placed in sealed quartz cuvettes of 1.0 cm path length with four polished windows.
Control samples of H 2 TPyP and Supra-H 2 TPyP dissolved in CHCl 3 were kept in a dark environment at room temperature (∼22.5°C) to evaluate their stability. Absorption spectra conrm that no spontaneous modications occur in both samples for 240.0 minutes of storage, endorsing their stability. This implies their durability is at least three times greater than the time demanded to perform the photo-induced reactions discussed herein.

Photophysics of Supra-H 2 TPyP
As shown in Fig. 2, the absorption spectrum of Supra-H 2 TPyP displays red-shied B-and Q-bands relative to H 2 TPyP, in agreement with literature. 16,60 Additionally, signatures belonging to the metallic complex are also observed (see intraligand bands located at around 311 nm). 47,61-65 Due to their superposition with H 2 TPyP bands, spectral signatures associated with the complex's MLCT bands centered around 470 nm are not resolved but lead to an overall increase of the spectrum baseline 49,52,56 (Fig. 2b).

Green-OPA irradiation of Supra-H 2 TPyP
In chloroform, H 2 TPyP is photostable under green-OPA irradiation. 15,16 This is not true for Supra-H 2 TPyP under the same irradiation condition. Fig. 3 shows that the spectroscopic signatures of Supra-H 2 TPyP are signicantly modied as a function of the incident uence. Because the modications occur in different stages, the results are separately discussed in three uence ranges: (i) 0-2400 J cm 2 ; (ii) 2400-6000 J cm 2 ; and (iii) 6000-19 200 J cm −2 .  In range (i), both B-and Q-bands undergo a blue shi. The main Supra-H 2 TPyP B-band, originally centered at 424 nm (0 J cm −2 ) (purple solid line), downshis to 419 nm at 2400 J cm −2 (olive solid line) while the Q y (0,1) downshis from 517 nm to 515 nm. This behavior suggests that Supra-H 2 TPyP is undergoing a similar photodissociation as reported for the H 2 TPyP [RuCl 2 (CO)(PPh 3 ) 2 ] 4 tetraruthenated porphyrin. 53 The occurrence of photodissociation in the uence range (i) would imply: (1) the decomposition of ruthenium complexes; and (2) the absence of processes associated with the uence ranges (ii) and (iii). 15,16 However, our data show that (1) and (2) do not happen, see Fig. 3. It is seen that in the uence range (i), the RuCl(dppb)(5,5 ′ -Me-bipy) complexes in Supra-H 2 TPyP, with intraligand bands spanning from ∼280-360 nm, undergo a sort of transformation, forming a novel supramolecular species, baptized PP-Supra-H 2 TPyP. The determination of the type of transformation that is photomodifying RuCl(dppb)(5,5 ′ -Mebipy) is out of the scope of this work but we understand that photooxidation, 66,67 photoisomerization, 68-70 and photoinduced ligand loss 71,72 are possible processes driving the phenomenon.
In range (ii), the peak at 419 nm no longer blueshis and starts developing into a novel spectroscopic signature centered at 444 nm, see Fig. 3b. The rise of this new signature occurs simultaneously with the enhancement of the Q x (0,0)-band (around 648 nm). These are signatures associated with photoprotonation of PP-Supra-H 2 TPyP. 15,16,[73][74][75] Moreover, the PP-Supra-H 2 TPyP 419 nm peak redshis in z2 nm, which is a signature for the protonation of the photomodied RuCl(dppb)(5,5 ′ -Me-bipy). 16,76 In fact, a closer inspection of RuCl(dppb)(5,5 ′ -Me-bipy) intraligand bands shows that the peak at ∼327 nm is enhanced (Fig. 3a). It is important to recall that protonation requires acid environments. This is reported for complexes similar to RuCl(dppb)(5,5 ′ -Me-bipy), in which an acidic solution was created through the addition of aliquots of HCl. 77,78 Since we do not add any acid to our solutions, the observation of protonation of both the porphyrin and the photomodied RuCl(dppb)(5,5 ′ -Me-bipy) conrms the release of HCl in the solution as a consequence of CHCl 3 photodecomposition via green-OPA.
In the range (iii), it is observed that protonated PP-Supra-H 2 TPyP signatures, B-(z444 nm) and Q x (0,0)-(z648 nm) bands, disappear, which is followed by the rise of new bands at z466 nm and z658 nm, respectively. The new features indicate that the protonated PP-Supra-H 2 TPyP is forming aggregates (possibly J-aggregates), similar to those previously reported for H 2 TPyP under ESA excitation. 16 It is also observed that the intraligand band of the photomodied RuCl(dppb)(5,5 ′ -Mebipy) at 327 nm undergoes a decrease in intensity with no indications of further transformations.
The protonation of Supra-H 2 TPyP also induces new signatures in the steady-state PL spectrum, which are in agreement with the results discussed above for UV-Vis absorption. Fig. 4a shows that the Supra-H 2 TPyP PL spectrum (purple solid line) evolves to the protonated PL spectrum (red solid line). 16 Aer protonation, the PL signal continuously decreases with increasing uence, becoming almost null at 19 200 J cm −2 . This decrease in the PL magnitude is associated with a reduction in the emission quantum yield of the protonated species, which is characteristic of J-aggregation in range (iii). 16,74,[79][80][81][82][83] Fig . 4b shows that the Supra-H 2 TPyP PL decay at 0 J cm −2 (purple solid line), which initially displays two characteristic lifetimes (4.54 ns (46%) and 2.01 ns (54%)), continuously evolves into the PP-Supra-H 2 TPyP decay prole at 2400 J cm −2 , with a dominant characteristic lifetime of z7.01 ns (91%). A minor contributing lifetime of z 3.17 ns (4%) is also measured and understood to belong to the remaining Supra-H 2 TPyP. In addition, a new PL decay with a characteristic lifetime of 0.98 ns (5%) associated with the protonated PP-Supra-H 2 TPyP is observed, see Table 2. The results in Fig. 4c and Table 2 support that in the range (i) both Supra-H 2 TPyP and PP-Supra-H 2 TPyP coexist. When the range (ii) starts (2400 J cm −2 ), PP-Supra-H 2 TPyP becomes predominant, coexisting with both Supra-H 2 TPyP and the protonated PP-Supra-H 2 TPyP. From 3600 J cm −2 on, the percentage contribution of the lifetime associated with Supra-H 2 TPyP becomes negligible.
As discussed in the literature, 15,16 free base porphyrins (H 2 TPyP and H 2 TPP) under UV-OPA (l exc = 266 nm) and ESA (l exc = 532 nm) excitations are able to photodecompose CHCl 3 and form HCl with excitation energy thresholds located above the B-band energy (z2.97 eV). 15,16 This implies that these freebase porphyrins never underwent photoprotonation under OPA operating at 475 nm (2.61 eV) or 532 nm (2.33 eV). 15,16 In this work, we were able to photoprotonate PP-Supra-H 2 TPyP dissolved in CHCl 3 under green-OPA (z2.33 eV), meaning HCl is being formed as a consequence of CHCl 3 decomposition. This process should not happen if the ground and the excited states of PP-Supra-H 2 TPyP were similar in energy to the ground and the excited states of H 2 TPyP. Since the excitation gap is xed by the CW laser energy employed in green-OPA (z2.33 eV), our results indicate two possible scenarios: (1) the involved ground state in PP-Supra-H 2 TPyP must be higher in energy in comparison to the correspondent state in H 2 TPyP and/or (2) the photooxidative excited state in PP-Supra-H 2 TPyP must be lower in energy concerning H 2 TPyP. Fig. 5a and b show the same photo-induced modications (PP-Supra-H 2 TPyP formation, photo-protonation, and photoaggregation) in the spectroscopic signatures of Supra-H 2 TPyP for green-ESA and red-OPA irradiations, respectively. The successful decomposition of CHCl 3 under red-OPA irradiation (photon energy delivered z1.96 eV) sets a new lower limit for the excitation energy required to trigger the process.

Other irradiation conditions
To understand the photodecomposition rates under the different laser irradiation conditions, the Supra-H 2 TPyP B-band integrated area (from 360 to 490 nm) for each laser absorbed uence (F A ) was normalized by the B-band integrated area (also from 360 to 490 nm) of the reference non-irradiated Supra-H 2 TPyP, and plotted as a function of F A (see Fig. 6). The  exponential proles obtained under each laser irradiation condition were tted using A F = e (−kF A ) , where A F and k represent, respectively, the aforementioned normalized areas for each F A and the net photomodication rate (given in cm 2 J −1 ). The tting results show that the rates obey the following hierarchy: green-ESA (k = 4.32 × 10 −3 cm 2 J −1 ) > green-OPA (k = 1.43 × 10 −3 cm 2 J −1 ) > red-OPA (k = 0.50 × 10 −3 cm 2 J −1 ), which shows that green-ESA leads to more efficient photoreaction processes in comparison with the OPA-based irradiations.

Conclusions
Supra-H 2 TPyP places itself as an excellent candidate for a broadrange OPA-visible-light active molecular photocatalyst in dyemediated chloroform decomposition. This supramolecular structure displays good photoreaction rates under visible OPA irradiation, which is a simpler and more affordable excitation mechanism than visible ESA and UV-OPA. Our results show that Supra-H 2 TPyP when combined with different excitation wavelengths can be used to controllably photodecompose CHCl 3 and, consequently, controllably photoprotonate Supra-H 2 TPyP itself, which is also interesting for sensing chloroform in solution and other applications in elds like materials science.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this manuscript.